Inert gas imaging (“IGI”) using hyperpolarized noble gases is a promising recent advance in Magnetic Resonance Imaging (MRI) and MR spectroscopy technologies. Conventionally, MRI has been used to produce images by exciting the nuclei of hydrogen molecules (present in water protons) in the human body. However, it has recently been discovered that polarized noble gases can produce improved images of certain areas and regions of the body which have heretofore produced less than satisfactory images in this modality. Polarized Helium-3 (“3He”) and Xenon-129 (“129Xe”) have been found to be particularly suited for this purpose. Unfortunately, as will be discussed further below, the polarized state of the gases is sensitive to handling and environmental conditions and can, undesirably, decay from the polarized state relatively quickly.
Various methods may be used to artificially enhance the polarization of certain noble gas nuclei (such as 129Xe or 3He) over the natural or equilibrium levels, i.e., the Boltzmann polarization. Such an increase is desirable because it enhances and increases the MRI signal intensity, allowing physicians to obtain better images of the substance in the body. See U.S. Pat. No. 5,545,396 to Albert et al., the disclosure of which is hereby incorporated by reference as if recited in full herein.
A “T1” decay time constant associated with the longitudinal relaxation of the hyperpolarized gas is often used to characterize the length of time it takes a gas sample to depolarize in a given situation. The handling of the hyperpolarized gas is critical because of the sensitivity of the hyperpolarized state to environmental and handling factors and thus the potential for undesirable decay of the gas from its hyperpolarized state prior to the planned end use, e.g., delivery to a patient for imaging. Processing, transporting, and storing the hyperpolarized gases—as well as delivering the gas to the patient or end user—can expose the hyperpolarized gases to various relaxation mechanisms such as magnetic field gradients, surface-induced relaxation, hyperpolarized gas atom interactions with other nuclei, paramagnetic impurities, and the like.
One way of minimizing the surface-induced decay of the hyperpolarized state is presented in U.S. Pat. No. 5,612,103 to Driehuys et al. entitled “Coatings for Production of Hyperpolarized Noble Gases.” Generally stated, this patent describes the use of a modified polymer as a surface coating on physical systems (such as a Pyrex™ container) which contact the hyperpolarized gas to inhibit the decaying effect of the surface of the collection chamber or storage unit. Other methods for minimizing surface or contact-induced decay are described in co-pending and co-assigned U.S. patent application Ser. No. 09/163,721 to Zollinger et al., entitled “Hyperpolarized Noble Gas Extraction Methods, Masking Methods, and Associated Transport Containers,” and co-pending and co-assigned U.S. Patent Application Ser. No. 09/334,400, entitled “Resilient Containers for Hyperpolarized Gases and Associated Methods.” The contents of these applications are hereby incorporated by reference as if recited in full herein.
However, other relaxation mechanisms arise during production, handling, storage, and transport of the hyperpolarized gas. These problems can be particularly troublesome when storing the gases (especially increased quantities) or transporting the hyperpolarized gas from a production site to a (remote) use site. In transit, the hyperpolarized gas can be exposed to many potentially depolarizing influences. In the past, a frozen amount of hyperpolarized 129Xe (about 300 cc–500 cc's) was collected in a cold finger and positioned in a metallic coated dewar along with a small yoke of permanent magnets arranged to provide a magnetic holding field therefor. The frozen gas was then taken to an experimental laboratory for delivery to an animal subject. Unfortunately, the permanent magnet yoke provided a relatively small magnetic field region (volume) with a relatively low magnetic homogeneity associated therewith. Further, the thawed sample yielded a relatively small amount of useful hyperpolarized 129Xe (used for small animal subjects) which would not generally be sufficient for most human sized patients.
There is, therefore, a need to provide improved ways to transport hyperpolarized gases so that the hyperpolarized gas is not unduly exposed to depolarizing effects during transport. Improved storage and transport methods and systems are desired so that the hyperpolarized product can retain sufficient polarization and larger amounts to allow effective imaging at delivery when stored or transported over longer transport distances in various (potentially depolarizing) environmental conditions, and for longer time periods from the initial polarization than has been viable previously.